U.S. patent number 10,985,730 [Application Number 16/996,381] was granted by the patent office on 2021-04-20 for filter devices having high power transversely-excited film bulk acoustic resonators.
This patent grant is currently assigned to Resonant Inc.. The grantee listed for this patent is Resonant Inc.. Invention is credited to Neal Fenzi, Bryant Garcia, Robert Hammond, Viktor Plesski, Patrick Turner, Ventsislav Yantchev.
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United States Patent |
10,985,730 |
Garcia , et al. |
April 20, 2021 |
Filter devices having high power transversely-excited film bulk
acoustic resonators
Abstract
There is disclosed acoustic resonators and filter devices. An
acoustic resonator includes a substrate having a surface and a
Z-cut piezoelectric plate having parallel front and back surfaces,
the back surface attached to the surface of the substrate except
for a portion of the piezoelectric plate forming a diaphragm that
spans a cavity in the substrate. An interdigital transducer (IDT)
is formed on the front surface of the single-crystal piezoelectric
plate such that interleaved fingers of the IDT are disposed on the
diaphragm. The IDT is configured to excite a primary acoustic mode
in the diaphragm in response to a radio frequency signal applied to
the IDT. A thickness of the interleaved fingers of the IDT is
greater than or equal to 0.85 times a thickness of the
diaphragm.
Inventors: |
Garcia; Bryant (Burlingame,
CA), Hammond; Robert (Santa Barbara, CA), Turner;
Patrick (San Bruno, CA), Fenzi; Neal (Santa Barbara,
CA), Plesski; Viktor (Gorgier, CH), Yantchev;
Ventsislav (Sofia, BG) |
Applicant: |
Name |
City |
State |
Country |
Type |
Resonant Inc. |
Goleta |
CA |
US |
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Assignee: |
Resonant Inc. (Austin,
TX)
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Family
ID: |
1000005502283 |
Appl.
No.: |
16/996,381 |
Filed: |
August 18, 2020 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20200382098 A1 |
Dec 3, 2020 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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16829617 |
Mar 25, 2020 |
10868512 |
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16578811 |
Apr 28, 2020 |
10637438 |
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16230443 |
Nov 26, 2019 |
10491192 |
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62685825 |
Jun 15, 2018 |
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62701363 |
Jul 20, 2018 |
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62741702 |
Oct 5, 2018 |
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62748883 |
Oct 22, 2018 |
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62753815 |
Oct 31, 2018 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H03H
9/02 (20130101); H03H 9/54 (20130101); H03H
9/171 (20130101); H03H 9/02015 (20130101); H03H
9/13 (20130101) |
Current International
Class: |
H03H
9/17 (20060101); H03H 9/54 (20060101); H03H
9/02 (20060101); H03H 9/13 (20060101) |
Field of
Search: |
;333/187 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2016017104 |
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Feb 2016 |
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WO |
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2018003273 |
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Jan 2018 |
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WO |
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Other References
R Olsson III, K. Hattar et al. "A high electromechanical coupling
coefficient SH0 Lamb wave lithiumniobate micromechanical resonator
and a method for fabrication" Sensors and Actuators A: Physical,
vol. 209, Mar. 1, 2014, pp. 183-190. cited by applicant .
M. Kadota, S. Tanaka, "Wideband acoustic wave resonators composed
of hetero acoustic layer structure," Japanese Journal of Applied
Physics, vol. 57, No. 7S1. Published Jun. 5, 2018. 5 pages. cited
by applicant .
Y. Yang, R. Lu et al. "Towards Ka Band Acoustics: Lithium Niobat
Asymmetrical Mode Piezoelectric MEMS Resonators", Department of
Electrical and Computer Engineering University of Illinois at
Urbana-Champaign, May 2018. pp. 1-2. cited by applicant .
Y. Yang, A. Gao et al. "5 GHZ Lithium Niobate MEMS Resonators With
High FOM of 153", 2017 IEEE 30th International Conference in Micro
Electro Mechanical Systems (MEMS). Jan. 22-26, 2017. pp. 942-945.
cited by applicant .
USPTO/ISA, International Search Report and Written Opinion for PCT
Application No. PCT/US2019/036433 dated Aug. 29, 2019. cited by
applicant .
USPTO/ISA, International Search Report and Written Opinion for PCT
Application No. PCT/US2019/058632 dated Jan. 17, 2020. cited by
applicant .
G. Manohar, "Investigation of Various Surface Acoustic Wave Design
Configurations for Improved Sensitivity." Doctoral dissertation,
University of South Florida, USA, Jan. 2012, 7 pages. cited by
applicant .
Ekeom, D. & Dubus, Bertrand & Volatier, A.. (2006). Solidly
mounted resonator (SMR) FEM-BEM simulation. 1474-1477.
10.1109/ULTSYM.2006.371. cited by applicant .
Mizutaui, K. and Toda, K., "Analysis of lamb wave propagation
characteristics in rotated Ycut Xpropagation LiNbO3 plates."
Electron. Comm. Jpn. Pt. I, 69, No. 4 (1986): 47-55.
doi:10.1002/ecja.4410690406. cited by applicant .
Naumenko et al., "Optimal orientations of Lithium Niobate for
resonator SAW filters", 2003 IEEE Ultrasonics Symposium--pp.
2110-2113. (Year: 2003). cited by applicant.
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Primary Examiner: Lee; Benny T
Assistant Examiner: Rahman; Hafizur
Attorney, Agent or Firm: SoCal IP Law Group LLP Gunther;
John E.
Parent Case Text
RELATED APPLICATION INFORMATION
This patent is a continuation of application Ser. No. 16/829,617,
filed Mar. 25, 2020, entitled HIGH POWER TRANSVERSELY-EXCITED FILM
BULK ACOUSTIC RESONATORS ON Z-CUT LITHIUM NIOB ATE, which has been
issued on Dec. 15, 2020 as a U.S. Pat. No. 10,868,512 and which is
a continuation of application Ser. No. 16/578,811, filed Sep. 23,
2019, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORS
FOR HIGH POWER APPLICATIONS, now U.S. Pat. No. 10,637,438 B2, which
is a continuation-in-part of application Ser. No. 16/230,443, filed
Dec. 21, 2018, entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC
RESONATOR, now U.S. Pat. No. 10,491,192 B2, which claims priority
from the following provisional patent applications: Application No.
62/685,825, filed Jun. 15, 2018, entitled SHEAR-MODE FBAR (XBAR);
Application No. 62/701,363, filed Jul. 20, 2018, entitled
SHEAR-MODE FBAR (XBAR); Application No. 62/741,702, filed Oct. 5,
2018, entitled 5 GHZ LATERALLY-EXCITED BULK WAVE RESONATOR (XBAR);
application 62/748,883, filed Oct. 22, 2018, entitled SHEAR-MODE
FILM BULK ACOUSTIC RESONATOR; and Application No. 62/753,815, filed
Oct. 31, 2018, entitled LITHIUM TANTALATE SHEAR-MODE FILM BULK
ACOUSTIC RESONATOR. All of these applications are incorporated
herein by reference.
Claims
It is claimed:
1. An acoustic resonator device comprising: a substrate having a
surface; a piezoelectric plate having front and back surfaces, the
back surface attached to the surface of the substrate except for a
portion of the piezoelectric plate forming a diaphragm that spans a
cavity in the substrate, the diaphragm having a thickness; and an
interdigital transducer (IDT) formed on the front surface of the
piezoelectric plate such that interleaved fingers of the IDT are
disposed on the diaphragm, the piezoelectric plate and the IDT
configured such that a radio frequency signal applied to the IDT
excites a shear primary acoustic mode in the diaphragm, wherein a
thickness of the interleaved fingers of the IDT is between 0.85 and
2.5 times the thickness of the diaphragm.
2. The acoustic resonator device of claim 1, wherein the
interleaved fingers of the IDT are substantially aluminum.
3. The acoustic resonator device of claim 2, further comprising a
front-side dielectric layer deposited between the fingers of the
IDT having a thickness greater than zero and less than or equal to
0.25 times the thickness of the diaphragm, wherein the thickness of
the interleaved fingers of the IDT is between 0.875 times and 2.25
times the thickness of the diaphragm.
4. The acoustic resonator device of claim 1, wherein the
interleaved fingers of the IDT are substantially copper and the
thickness of the interleaved fingers of the IDT is between 0.85 and
1.42 times the thickness of the diaphragm, or between 1.95 and
2.325 times the thickness of the diaphragm.
5. The acoustic resonator device of claim 4, further comprising a
front-side dielectric layer deposited between the fingers of the
IDT, a thickness of the front-side dielectric layer being greater
than zero and less than or equal to 100 nm, wherein the thickness
of the interleaved fingers of the IDT is between 0.85 and 1.42
times the thickness of the diaphragm.
6. The acoustic resonator device of claim 1, wherein the thickness
of the diaphragm is between 300 nm and 500 nm.
7. The acoustic resonator device of claim 1, wherein a pitch of the
interleaved fingers of the IDT is between 6 and 12.5 times the
thickness of the diaphragm.
8. The acoustic resonator device of claim 1, wherein an aperture of
the IDT is between 20 and 60 microns.
9. The acoustic resonator device of claim 1, wherein a direction of
acoustic energy flow of the primary acoustic mode is substantially
normal to the front and back surfaces of the diaphragm.
10. The acoustic resonator device of claim 1, wherein the diaphragm
is contiguous with the piezoelectric plate around at least 50% of a
perimeter of the cavity.
11. A filter device, comprising: a substrate; a piezoelectric plate
having front and back surfaces, the back surface attached to the
surface of the substrate, portions of the piezoelectric plate
forming one or more diaphragms spanning respective cavities in the
substrate, the diaphragms having a common thickness; and a
conductor pattern formed on the front surface, the conductor
pattern including a plurality of interdigital transducers (IDTs) of
a respective plurality of acoustic resonators, interleaved fingers
of each of the plurality of IDTs disposed on a diaphragm of the one
or more diaphragms, the piezoelectric plate and all of the IDTs
configured such that respective radio frequency signals applied to
each IDT excite respective shear primary acoustic modes in the
respective diaphragms, wherein the interleaved fingers of all of
the plurality of IDTs have a common finger thickness which between
0.85 and 2.5 times the thickness of the diaphragms.
12. The filter device of claim 11, wherein the interleaved fingers
of all of the plurality of IDTs are substantially aluminum.
13. The filter device of claim 12, further comprising a front-side
dielectric layer deposited between the fingers of at least one of
the plurality of IDTs, a thickness of the front-side dielectric
layer being greater than zero and less than or equal to 0.25 times
the thickness of the diaphragms, wherein the common finger
thickness is between 0.875 and 2.25 times the thickness of the
diaphragms.
14. The filter device of claim 11, wherein the interleaved fingers
of all of the plurality of IDTs are substantially copper, and the
common finger thickness is between 0.85 and 1.42 times the
thickness of the diaphragms.
15. The filter device of claim 14, further comprising a front-side
dielectric layer deposited between the fingers of at least one of
the plurality of IDTs, a thickness of the front-side dielectric
layer being greater than zero and less than or equal to 0.25 times
the thickness of the diaphragms.
16. The filter device of claim 11, wherein the thickness of the
diaphragms is greater than or equal to 300 nm and less than or
equal to 500 nm.
17. The filter device of claim 11, wherein respective pitches of
the interleaved fingers of all of the plurality of IDTs are between
6 and 12.5 times the thickness of the diaphragms.
18. The filter device of claim 11, wherein respective apertures of
all of the plurality of IDTs are between 20 and 60 microns.
19. The filter device of claim 11, wherein a direction of acoustic
energy flow of the respective primary acoustic modes excited by all
of the IDTs is substantially normal to the front and back surfaces
of the diaphragm.
20. The filter device of claim 11, wherein each diaphragm of the
one or more diaphragms is contiguous with the piezoelectric plate
around at least 50% of a perimeter of the respective cavity.
21. A filter device, comprising: a substrate; a piezoelectric plate
having front and back surfaces, the back surface attached to the
surface of the substrate, portions of the piezoelectric plate
forming one or more diaphragms spanning respective cavities in the
substrate, the diaphragms having a common thickness; a conductor
pattern formed on the front surface, the conductor pattern
including a plurality of interdigital transducers (IDTs) of a
respective plurality of acoustic resonators, interleaved fingers of
each of the plurality of IDTs disposed on a diaphragm of the one or
more diaphragms, the plurality of resonators including one or more
shunt resonators and one or more series resonators; a first
dielectric layer having a first thickness deposited between the
fingers of the IDTs of the one or more shunt resonators; and a
second dielectric layer having a second thickness deposited between
the fingers of the IDTs of the one or more series resonators,
wherein the second thickness is less than the first thickness and
greater than or equal to zero, and the interleaved fingers of all
of the plurality of IDTs have a common finger thickness which is
between 0.875 and 2.25 times the thickness of the diaphragms.
22. The filter device of claim 21, wherein the interleaved fingers
of all of the plurality of IDTs are substantially aluminum.
23. The filter device of claim 21, wherein the interleaved fingers
of all of the plurality of IDTs are substantially copper, and the
common finger thickness is between 0.85 and 1.42 times the
thickness of the diaphragms.
24. The filter device of claim 21, wherein the thickness of the
diaphragms is between 300 nm and 500 nm.
25. The filter device of claim 21, wherein respective pitches of
the interleaved fingers of all of the plurality of IDTs are between
6 and 12.5 times the thickness of the diaphragms.
26. The filter device of claim 21, wherein respective apertures of
all of the plurality of IDTs are between 20 and 60 microns.
27. The filter device of claim 21, wherein a direction of acoustic
energy flow of the respective primary acoustic modes excited by all
of the plurality of IDTs is substantially orthogonal to the front
and back surfaces of the diaphragm.
28. The filter device of claim 21, wherein each diaphragm of the
one or more diaphragms is contiguous with the piezoelectric plate
around at least 50% of a perimeter of the respective cavity.
29. The filter device of claim 21, wherein the first thickness is
less than or equal to 0.25 times the thickness of the diaphragms.
Description
NOTICE OF COPYRIGHTS AND TRADE DRESS
A portion of the disclosure of this patent document contains
material which is subject to copyright protection. This patent
document may show and/or describe matter which is or may become
trade dress of the owner. The copyright and trade dress owner has
no objection to the facsimile reproduction by anyone of the patent
disclosure as it appears in the Patent and Trademark Office patent
files or records, but otherwise reserves all copyright and trade
dress rights whatsoever.
BACKGROUND
Field
This disclosure relates to radio frequency filters using acoustic
wave resonators, and specifically to bandpass filters with high
power capability for use in communications equipment.
Description of the Related Art
A radio frequency (RF) filter is a two-port device configured to
pass some frequencies and to stop other frequencies, where "pass"
means transmit with relatively low signal loss and "stop" means
block or substantially attenuate. The range of frequencies passed
by a filter is referred to as the "pass-band" of the filter. The
range of frequencies stopped by such a filter is referred to as the
"stop-band" of the filter. A typical RF filter has at least one
pass-band and at least one stop-band. Specific requirements on a
pass-band or stop-band depend on the specific application. For
example, a "pass-band" may be defined as a frequency range where
the insertion loss of a filter is less than a defined value such as
1 dB, 2 dB, or 3 dB. A "stop-band" may be defined as a frequency
range where the rejection of a filter is greater than a defined
value such as 20 dB, 30 dB, 40 dB, or greater depending on
application.
RF filters are used in communications systems where information is
transmitted over wireless links. For example, RF filters may be
found in the RF front-ends of cellular base stations, mobile
telephone and computing devices, satellite transceivers and ground
stations, IoT (Internet of Things) devices, laptop computers and
tablets, fixed point radio links, and other communications systems.
RF filters are also used in radar and electronic and information
warfare systems.
RF filters typically require many design trade-offs to achieve, for
each specific application, the best compromise between performance
parameters such as insertion loss, rejection, isolation, power
handling, linearity, size and cost. Specific design and
manufacturing methods and enhancements can benefit simultaneously
one or several of these requirements.
Performance enhancements to the RF filters in a wireless system can
have broad impact to system performance. Improvements in RF filters
can be leveraged to provide system performance improvements such as
larger cell size, longer battery life, higher data rates, greater
network capacity, lower cost, enhanced security, higher
reliability, etc. These improvements can be realized at many levels
of the wireless system both separately and in combination, for
example at the RF module, RF transceiver, mobile or fixed
sub-system, or network levels.
High performance RF filters for present communication systems
commonly incorporate acoustic wave resonators including surface
acoustic wave (SAW) resonators, bulk acoustic wave (BAW)
resonators, film bulk acoustic wave resonators (FBAR), and other
types of acoustic resonators. However, these existing technologies
are not well-suited for use at the higher frequencies proposed for
future communications networks.
The desire for wider communication channel bandwidths will
inevitably lead to the use of higher frequency communications
bands. Radio access technology for mobile telephone networks has
been standardized by the 3GPP (3.sup.rd Generation Partnership
Project). Radio access technology for 5.sup.th generation mobile
networks is defined in the 5G NR (new radio) standard. The 5G NR
standard defines several new communications bands. Two of these new
communications bands are n77, which uses the frequency range from
3300 MHz to 4200 MHz, and n79, which uses the frequency range from
4400 MHz to 5000 MHz. Both band n77 and band n79 use time-division
duplexing (TDD), such that a communications device operating in
band n77 and/or band n79 use the same frequencies for both uplink
and downlink transmissions. Bandpass filters for bands n77and n79
must be capable of handling the transmit power of the
communications device. The 5G NR standard also defines millimeter
wave communication bands with frequencies between 24.25 GHz and 40
GHz.
DESCRIPTION OF THE DRAWINGS
FIG. 1 includes a schematic plan view and two schematic
cross-sectional views of a transversely-excited film bulk acoustic
resonator (XBAR).
FIG. 2 is an expanded schematic cross-sectional view of a portion
of the XBAR of FIG. 1.
FIG. 3A is an alternative schematic cross-sectional view of the
XBAR of FIG. 1.
FIG. 3B is another alternative schematic cross-sectional view of
the XBAR of FIG. 1.
FIG. 3C is an alternative schematic plan view of an XBAR
FIG. 4 is a graphic illustrating a primary acoustic mode in an
XBAR.
FIG. 5 is a schematic circuit diagram of a band-pass filter using
acoustic resonators in a ladder circuit.
FIG. 6 is a graph showing the relationship between piezoelectric
diaphragm thickness and resonance frequency of an XBAR.
FIG. 7 is a plot showing the relationship between coupling factor
Gamma (F) and IDT pitch for an XBAR.
FIG. 8 is a graph showing the dimensions of XBAR resonators with
capacitance equal to one picofarad.
FIG. 9 is a graph showing the relationship between IDT finger pitch
and resonance and anti-resonance frequencies of an XBAR, with
dielectric layer thickness as a parameter.
FIG. 10 is a graph comparing the admittances of three simulated
XBARs with different IDT metal thicknesses.
FIG. 11 is a graph illustrating the effect of IDT finger width on
spurious resonances in an XBAR.
FIG. 12 is a graph identifying preferred combinations of aluminum
IDT thickness and IDT pitch for XBARs without a front dielectric
layer.
FIG. 13 is a graph identifying preferred combinations of aluminum
IDT thickness and IDT pitch for XBARs with front dielectric layer
thickness equal to 0.25 times the XBAR diaphragm thickness.
FIG. 14 is a graph identifying preferred combinations of copper IDT
thickness and IDT pitch for XBARs without a front dielectric
layer.
FIG. 15 is a graph identifying preferred combinations of copper IDT
thickness and IDT pitch for XBARs with front dielectric layer
thickness equal to 0.25 times the XBAR diaphragm thickness.
FIG. 16 is a graph identifying preferred combinations of aluminum
IDT thickness and IDT pitch for XBARs without a front dielectric
layer for diaphragm thicknesses of 300 nm, 400 nm, and 500 nm.
FIG. 17 is a detailed cross-section view of a portion of the XBAR
100 of FIG. 1.
FIG. 18 is a schematic circuit diagram of an exemplary high-power
band-pass filter using XBARs.
FIG. 19 is a layout of the filter of FIG. 18.
FIG. 20 is a graph of measured S-parameters S11 and S21 versus
frequency for the filter of FIG. 18 and FIG. 19.
FIG. 21 is a graph of measured S-parameters S11 and S21 versus
frequency, over a wider frequency range, for the filter of FIG. 18
and FIG. 19.
Throughout this description, elements appearing in figures are
assigned three-digit or four-digit reference designators, where the
two least significant digits are specific to the element and the
one or two most significant digit is the figure number where the
element is first introduced. An element that is not described in
conjunction with a figure may be presumed to have the same
characteristics and function as a previously-described element
having the same reference designator.
DETAILED DESCRIPTION
Description of Apparatus
FIG. 1 shows a simplified schematic top view and orthogonal
cross-sectional views of a transversely-excited film bulk acoustic
resonator (XBAR) 100. XBAR resonators such as the resonator 100 may
be used in a variety of RF filters including band-reject filters,
band-pass filters, duplexers, and multiplexers. XBARs are
particularly suited for use in filters for communications bands
with frequencies above 3 GHz.
The XBAR 100 is made up of a thin film conductor pattern formed on
a surface of a piezoelectric plate 110 having parallel front and
back surfaces 112, 114, respectively. The piezoelectric plate is a
thin single-crystal layer of a piezoelectric material such as
lithium niobate, lithium tantalate, lanthanum gallium silicate,
gallium nitride, or aluminum nitride. The piezoelectric plate is
cut such that the orientation of the X, Y, and Z crystalline axes
with respect to the front and back surfaces is known and
consistent. In the examples presented in this patent, the
piezoelectric plates are Z-cut, which is to say the Z axis is
normal to the front and back surfaces 112, 114. However, XBARs may
be fabricated on piezoelectric plates with other crystallographic
orientations.
The back surface 114 of the piezoelectric plate 110 is attached to
a surface of the substrate 120 except for a portion of the
piezoelectric plate 110 that forms a diaphragm 115 spanning a
cavity 140 formed in the substrate. The portion of the
piezoelectric plate that spans the cavity is referred to herein as
the "diaphragm" 115 due to its physical resemblance to the
diaphragm of a microphone. As shown in FIG. 1, the diaphragm 115 is
contiguous with the rest of the piezoelectric plate 110 around all
of a perimeter 145 of the cavity 140. In this context, "contiguous"
means "continuously connected without any intervening item". In
other configurations, the diaphragm 115 may be contiguous with the
piezoelectric plate are at least 50% of the perimeter 145 of the
cavity 140.
The substrate 120 provides mechanical support to the piezoelectric
plate 110. The substrate 120 may be, for example, silicon,
sapphire, quartz, or some other material or combination of
materials. The back surface 114 of the piezoelectric plate 110 may
be bonded to the substrate 120 using a wafer bonding process.
Alternatively, the piezoelectric plate 110 may be grown on the
substrate 120 or attached to the substrate in some other manner.
The piezoelectric plate 110 may be attached directly to the
substrate or may be attached to the substrate 120 via one or more
intermediate material layers (not shown in FIG. 1).
"Cavity" has its conventional meaning of "an empty space within a
solid body." The cavity 140 may be a hole completely through the
substrate 120 (as shown in Section A-A and Section B-B) or a recess
in the substrate 120 under the diaphragm 115. The cavity 140 may be
formed, for example, by selective etching of the substrate 120
before or after the piezoelectric plate 110 and the substrate 120
are attached.
The conductor pattern of the XBAR 100 includes an interdigital
transducer (IDT) 130. The IDT 130 includes a first plurality of
parallel fingers, such as finger 136, extending from a first busbar
132 and a second plurality of fingers extending from a second
busbar 134. The first and second pluralities of parallel fingers
are interleaved. The interleaved fingers overlap for a distance AP,
commonly referred to as the "aperture" of the IDT. The
center-to-center distance L between the outermost fingers of the
IDT 130 is the "length" of the IDT.
The first and second busbars 132, 134 serve as the terminals of the
XBAR 100. A radio frequency or microwave signal applied between the
two busbars 132, 134 of the IDT 130 excites a primary acoustic mode
within the piezoelectric plate 110. As will be discussed in further
detail, the primary acoustic mode is a bulk shear mode where
acoustic energy propagates along a direction substantially
orthogonal to the surface of the piezoelectric plate 110, which is
also normal, or transverse, to the direction of the electric field
created by the IDT fingers. Thus, the XBAR is considered a
transversely-excited film bulk wave resonator.
The IDT 130 is positioned on the piezoelectric plate 110 such that
at least the fingers of the IDT 130 are disposed on the portion 115
of the piezoelectric plate that spans, or is suspended over, the
cavity 140. As shown in FIG. 1, the cavity 140 has a rectangular
shape with an extent greater than the aperture AP and length L of
the IDT 130. A cavity of an XBAR may have a different shape, such
as a regular or irregular polygon. The cavity of an XBAR may more
or fewer than four sides, which may be straight or curved.
For ease of presentation in FIG. 1, the geometric pitch and width
of the IDT fingers is greatly exaggerated with respect to the
length (dimension L) and aperture (dimension AP) of the XBAR. A
typical XBAR has more than ten parallel fingers in the IDT 110. An
XBAR may have hundreds, possibly thousands, of parallel fingers in
the IDT 110. Similarly, the thickness of the fingers in the
cross-sectional views is greatly exaggerated.
FIG. 2 shows a detailed schematic cross-sectional view of the XBAR
100. The piezoelectric plate 110 is a single-crystal layer of
piezoelectrical material having a thickness ts. ts may be, for
example, 100 nm to 1500 nm. When used in filters for LTE bands from
3.4 GHZ to 6 GHz (e.g. bands 42, 43, 46), the thickness ts may be,
for example, 200 nm to 1000 nm.
A front-side dielectric layer 214 may optionally be formed on the
front side of the piezoelectric plate 110. The "front side" of the
XBAR is, by definition, the surface facing away from the substrate.
The front-side dielectric layer 214 has a thickness tfd. The
front-side dielectric layer 214 may be formed only between the IDT
fingers (e.g. IDT finger 238b) or may be deposited as a blanket
layer such that the dielectric layer is formed both between and
over the IDT fingers (e.g. IDT finger 238a). The front-side
dielectric layer 214 may be a non-piezoelectric dielectric
material, such as silicon dioxide or silicon nitride. tfd may be,
for example, 0 to 500 nm. tfd is typically less than the thickness
ts of the piezoelectric plate. The front-side dielectric layer 214
may be formed of multiple layers of two or more materials.
The IDT fingers 238 may be aluminum, an aluminum alloy, copper, a
copper alloy, beryllium, gold, tungsten, molybdenum or some other
conductive material. The IDT fingers are considered to be
"substantially aluminum" if they are formed from aluminum or an
alloy comprising at least 50% aluminum. The IDT fingers are
considered to be "substantially copper" if they are formed from
copper or an alloy comprising at least 50% copper. Thin (relative
to the total thickness of the conductors) layers of other metals,
such as chromium or titanium, may be formed under and/or over
and/or as layers within the fingers to improve adhesion between the
fingers and the piezoelectric plate 110 and/or to passivate or
encapsulate the fingers and/or to improve power handling. The
busbars (132, 134 in FIG. 1) of the IDT may be made of the same or
different materials as the fingers.
Dimension p is the center-to-center spacing or "pitch" of the IDT
fingers, which may be referred to as the pitch of the IDT and/or
the pitch of the XBAR. Dimension w is the width or "mark" of the
IDT fingers. The geometry of the IDT of an XBAR differs
substantially from the IDTs used in surface acoustic wave (SAW)
resonators. In a SAW resonator, the pitch of the IDT is one-half of
the acoustic wavelength at the resonance frequency. Additionally,
the mark-to-pitch ratio of a SAW resonator IDT is typically close
to 0.5 (i.e. the mark or finger width is about one-fourth of the
acoustic wavelength at resonance). In an XBAR, the pitch p of the
IDT is typically 2 to 20 times the width w of the fingers. In
addition, the pitch p of the IDT is typically 2 to 20 times the
thickness is of the piezoelectric slab 212. The width of the IDT
fingers in an XBAR is not constrained to be near one-fourth of the
acoustic wavelength at resonance. For example, the width of XBAR
IDT fingers may be 500 nm or greater, such that the IDT can be
readily fabricated using optical lithography. The thickness tm of
the IDT fingers may be from 100 nm to about equal to the width w.
The thickness of the busbars (132, 134 in FIG. 1) of the IDT may be
the same as, or greater than, the thickness tm of the IDT
fingers.
FIG. 3A and FIG. 3B show two alternative cross-sectional views
A'-A' and A''-A'' along the section plane A-A defined in FIG. 1. In
FIG. 3A, a piezoelectric plate 310 is attached to a substrate 320.
A portion of the piezoelectric plate 310 forms a diaphragm 315
spanning a cavity 340 in the substrate. The cavity 340 does not
fully penetrate the substrate 320. Fingers of an IDT are disposed
on the diaphragm 315. The cavity 340 may be formed, for example, by
etching the substrate 320 before attaching the piezoelectric plate
310. Alternatively, the cavity 340 may be formed by etching the
substrate 320 with a selective etchant that reaches the substrate
through one or more openings (not shown) provided in the
piezoelectric plate 310. In this case, the diaphragm 315 may be
contiguous with the rest of the piezoelectric plate 310 around a
large portion of a perimeter 345 of the cavity 340. For example,
the diaphragm 315 may be contiguous with the rest of the
piezoelectric plate 310 around at least 50% of the perimeter 345 of
the cavity 340. An intermediate layer (not shown), such as a
dielectric bonding layer, may be present between the piezo electric
plate 340 and the substrate 320.
In FIG. 3B, the substrate 320 includes a base 322 and an
intermediate layer 324 disposed between the piezoelectric plate 310
and the base 322. For example, the base 322 may be silicon and the
intermediate layer 324 may be silicon dioxide or silicon nitride or
some other material. A portion of the piezoelectric plate 310 forms
a diaphragm 315 spanning a cavity 340 in the intermediate layer
324. Fingers of an IDT are disposed on the diaphragm 315. The
cavity 340 may be formed, for example, by etching the intermediate
layer 324 before attaching the piezoelectric plate 310.
Alternatively, the cavity 340 may be formed by etching the
intermediate layer 324 with a selective etchant that reaches the
substrate through one or more openings provided in the
piezoelectric plate 310. In this case, the diaphragm 315 may be
contiguous with the rest of the piezoelectric plate 310 around a
large portion of a perimeter 345 of the cavity 340. For example,
the diaphragm 315 may be contiguous with the rest of the
piezoelectric plate 310 around at least 50% of the perimeter 345 of
the cavity 340 as shown in FIG. 3C. Although not shown in FIG. 3B,
a cavity formed in the intermediate layer 324 may extend into the
base 322.
FIG. 3C is a schematic plan view of another XBAR 350. The XBAR 350
includes an IDT formed on a piezoelectric plate 310. A portion of
the piezoelectric plate 310 forms a diaphragm spanning a cavity in
a substrate. In this example, the perimeter 345 of the cavity has
an irregular polygon shape such that none of the edges of the
cavity are parallel, nor are they parallel to the conductors of the
IDT. A cavity may have a different shape with straight or curved
edges.
FIG. 4 is a graphical illustration of the primary acoustic mode of
interest in an XBAR. FIG. 4 shows a small portion of an XBAR 400
including a piezoelectric plate 410 and three interleaved IDT
fingers 430 which alternate in electrical polarity from finger to
finger. An RF voltage is applied to the interleaved fingers 430.
This voltage creates a time-varying electric field between the
fingers. The direction of the electric field is predominantly
lateral, or parallel to the surface of the piezoelectric plate 410,
as indicated by the arrows labeled "electric field". Due to the
high dielectric constant of the piezoelectric plate, the RF
electric energy is highly concentrated inside the plate relative to
the air. The lateral electric field introduces shear deformation
which couples strongly to a shear primary acoustic mode (at a
resonance frequency defined by the acoustic cavity formed by the
volume between the two surfaces of the piezoelectric plate) in the
piezoelectric plate 410. In this context, "shear deformation" is
defined as deformation in which parallel planes in a material
remain predominantly parallel and maintain constant separation
while translating (within their respective planes) relative to each
other. A "shear acoustic mode" is defined as an acoustic vibration
mode in a medium that results in shear deformation of the medium.
The shear deformations in the XBAR 400 are represented by the
curves 460, with the adjacent small arrows providing a schematic
indication of the direction and relative magnitude of atomic motion
at the resonance frequency. The degree of atomic motion, as well as
the thickness of the piezoelectric plate 410, have been greatly
exaggerated for ease of visualization. While the atomic motions are
predominantly lateral (i.e. horizontal as shown in FIG. 4), the
direction of acoustic energy flow of the excited primary acoustic
mode is substantially orthogonal to the surface of the
piezoelectric plate, as indicated by the arrow 465.
Considering FIG. 4, there is essentially no RF electric field
immediately under the IDT fingers 430, and thus acoustic modes are
only minimally excited in the regions 470 under the fingers. There
may be evanescent acoustic motions in these regions. Since acoustic
vibrations are not excited under the IDT fingers 430, the acoustic
energy coupled to the IDT fingers 430 is low (for example compared
to the fingers of an IDT in a SAW resonator) for the primary
acoustic mode, which minimizes viscous losses in the IDT
fingers.
An acoustic resonator based on shear acoustic wave resonances can
achieve better performance than current state-of-the art
film-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonator
bulk-acoustic-wave (SMR BAW) devices where the electric field is
applied in the thickness direction. In such devices, the acoustic
mode is compressive with atomic motions and the direction of
acoustic energy flow in the thickness direction. In addition, the
piezoelectric coupling for shear wave XBAR resonances can be high
(>20%) compared to other acoustic resonators. High piezoelectric
coupling enables the design and implementation of microwave and
millimeter-wave filters with appreciable bandwidth.
FIG. 5 is a schematic circuit diagram of a band-pass filter 500
using five XBARs X1-X5. The filter 500 may be, for example, a band
n79 band-pass filter for use in a communication device. The filter
500 has a conventional ladder filter architecture including three
series resonators X1, X3, X5 and two shunt resonators X2, X4. The
three series resonators X1, X3, X5 are connected in series between
a first port and a second port. In FIG. 5, the first and second
ports are labeled "In" and "Out", respectively. However, the filter
500 is symmetrical and either port may serve as the input or output
of the filter. The two shunt resonators X2, X4 are connected from
nodes between the series resonators to ground. All the shunt
resonators and series resonators are XBARs.
The three series resonators X1, X3, X5 and the two shunt resonators
X2, X4 of the filter 500 maybe formed on a single plate 530 of
piezoelectric material bonded to a silicon substrate (not visible).
Each resonator includes a respective IDT (not shown), with at least
the fingers of the IDT disposed over a cavity in the substrate. In
this and similar contexts, the term "respective" means "relating
things each to each", which is to say with a one-to-one
correspondence. In FIG. 5, the cavities are illustrated
schematically as the dashed rectangles (such as the rectangle 535).
In this example, an IDT of each resonator is disposed over a
respective cavity. In other filters, the IDTs of two or more
resonators may be disposed over a common cavity. Resonators may
also be cascaded into multiple IDTs which may be formed on multiple
cavities.
Each of the resonators X1 to X5 has a resonance frequency and an
anti-resonance frequency. In over-simplified terms, each resonator
is effectively a short circuit at its resonance frequency and
effectively an open circuit at its anti-resonance frequency. Each
resonator X1 to X5 creates a "transmission zero", where the
transmission between the in and out ports of the filter is very
low. Note that the transmission at a "transmission zero" is not
actually zero due to energy leakage through parasitic components
and other effects. The three series resonators X1, X3, X5 create
transmission zeros at their respective anti-resonance frequencies
(where each resonator is effectively an open circuit). The two
shunt resonators X2, X4 create transmission zeros at their
respective resonance frequencies (where each resonator is
effectively a short circuit). In a typical band-pass filter using
acoustic resonators, the anti-resonance frequencies of the series
resonators are above the passband, and the resonance frequencies of
the shunt resonators are below the passband.
A band-pass filter for use in a communications device, such as a
cellular telephone, must meet a variety of requirements. First, a
band-pass filter, by definition, must pass, or transmit with
acceptable loss, a defined pass-band. Typically, a band-pass filter
for use in a communications device must also stop, or substantially
attenuate, one or more stop band(s). For example, a band n79
band-pass filter is typically required to pass the n79 frequency
band from 4400 MHz to 5000 MHz and to stop the 5 GHz WiFi.TM. band
and/or the n77 band from 3300 MHz to 4200 MHz. To meet these
requirements, a filter using a ladder circuit would require series
resonators with anti-resonance frequencies about or above 5100 MHz,
and shunt resonators with resonance frequencies about or below 4300
MHz.
The resonance and anti-resonance frequencies of an XBAR are
strongly dependent on the thickness is of the piezoelectric
membrane (115 in FIG. 1). FIG. 6 is a graph 600 of resonance
frequency of an XBAR versus piezoelectric diaphragm thickness. In
this example, the piezoelectric diaphragm is z-cut lithium niobate.
The solid curve 610 is plot of resonance frequency as function of
the inverse of the piezoelectric plate thickness for XBARs with IDT
pitch equal to 3 microns. This plot is based on results of
simulations of XBARs using finite element methods. The resonance
frequency is roughly proportional to the inverse of the
piezoelectric plate thickness.
The resonance and anti-resonance frequencies of an XBAR are also
dependent on the pitch (dimension p in FIG. 2) of the IDT. Further,
the electromechanical coupling of an XBAR, which determines the
separation between the resonance and anti-resonance frequencies, is
dependent on the pitch. FIG. 7 is a graph of gamma (.GAMMA.) as a
function of normalized pitch, which is to say IDT pitch p divided
by diaphragm thickness ts. Gamma is a metric defined by the
equation:
.GAMMA..times..times..times. ##EQU00001## where Fa is the
antiresonance frequency and Fr is the resonance frequency. Large
values for gamma correspond to smaller separation between the
resonance and anti-resonance frequencies. Low values of gamma
indicate strong coupling which is good for wideband ladder
filters.
In this example, the piezoelectric diaphragm is z-cut lithium
niobate, and data is presented for diaphragm thicknesses of 300 nm,
400 nm, and 500 nm. In all cases the IDT is aluminum with a
thickness of 25% of the diaphragm thickness, the duty factor (i.e.
the ratio of the width w to the pitch p) of the IDT fingers is
0.14, and there are no dielectric layers. The "+" symbols, circles,
and ".times." symbols represent diaphragm thicknesses of 300 nm,
400 nm, and 500 nm, respectively. Outlier data points, such as
those for relative IDT pitch about 4.5 and about 8, are caused by
spurious modes interacting with the primary acoustic mode and
altering the apparent gamma. The relationship between gamma and IDT
pitch is relatively independent of diaphragm thickness, and roughly
asymptotic to .GAMMA.=3.5 as the relative pitch is increased.
Another typical requirement on a band-pass filter for use in a
communications device is the input and output impedances of the
filter have to match, at least over the pass-band of the filter,
the impedances of other elements of the communications device to
which the filter is connected (e.g. a transmitter, receiver, and/or
antenna) for maximum power transfer. Commonly, the input and output
impedances of a band-pass filter are required to match a 50-ohm
impedance within a tolerance that may be expressed, for example, as
a maximum return loss or a maximum voltage standing wave ratio.
When necessary, an impedance matching network comprising one or
more reactive components can be used at the input and/or output of
a band-pass filter. Such impedance matching networks add to the
complexity, cost, and insertion loss of the filter and are thus
undesirable. To match, without additional impedance matching
components, a 50-Ohm impedance at a frequency of 5 GHz, the
capacitances of at least the shunt resonators in the band-pass
filter need to be in a range of about 0.5 picofarads (pF) to about
1.5 picofarads.
FIG. 8 is a graph showing the area and dimensions of XBAR
resonators with capacitance equal to one picofarad. The solid line
810 is a plot of the IDT length required provide a capacitance of 1
pF as a function of the inverse of the IDT aperture when the IDT
pitch is 3 microns. The dashed line 820 is a plot of the IDT length
required provide a capacitance of 1 pF as a function of the inverse
of the IDT aperture when the IDT pitch is 5 microns. The data
plotted in FIG. 8 is specific to XBAR devices with lithium niobate
diaphragm thickness of 400 nm.
For any aperture, the IDT length required to provide a desired
capacitance is greater for an IDT pitch of 5 microns than for an
IDT pitch of 3 microns. The required IDT length is roughly
proportional to the change in IDT pitch. The design of a filter
using XBARs is a compromise between somewhat conflicting
objectives. As shown in FIG. 7, a larger IDT pitch may be preferred
to reduce gamma and maximize the separation between the
anti-resonance and resonance frequencies. As can be understood from
FIG. 8, smaller IDT pitch is preferred to minimize IDT area. A
reasonable compromise between these objectives is
6.ltoreq.p/ts.ltoreq.12.5. Setting the IDT pitch p equal to or
greater than six times the diaphragm thickness ts provides Fa/Fr
greater than 1.1. Setting the maximum IDT pitch p to 12.5 times the
diaphragm thickness ts is reasonable since Fa/Fr does not increase
appreciably for higher values of relative pitch.
As will be discussed is greater detail subsequently, the metal
fingers of the IDTs provide the primary mechanism for removing heat
from an XBAR resonator. Increasing the aperture of a resonator
increases the length and the electrical and thermal resistance of
each IDT finger. Further, for a given IDT capacitance, increasing
the aperture reduces the number of fingers required in the IDT,
which, in turn, proportionally increases the RF current flowing in
each finger. All of these effects argue for using the smallest
possible aperture in resonators for high-power filters.
Conversely, several factors argue for using a large aperture.
First, the total area of an XBAR resonator includes the area of the
IDT and the area of the bus bars. The area of the bus bars is
generally proportional to the length of the IDT. For very small
apertures, the area of the IDT bus bars may be larger than the area
occupied by the interleaved IDT fingers. Further, some electrical
and acoustic energy may be lost at the ends of the IDT fingers.
These loss effects become more significant as IDT aperture is
reduced and the total number of fingers is increased. These losses
may be evident as a reduction in resonator Q-factor, particularly
at the anti-resonance frequency, as IDT aperture is reduced.
As a compromise between conflicting objectives, resonators
apertures will typically fall in the range from 20 .mu.m and 60
.mu.m.
The resonance and anti-resonance frequencies of an XBAR are also
dependent on the thickness (dimension tfd in FIG. 2) of the
front-side dielectric layer applied between (and optionally over)
the fingers of the IDT. FIG. 9 is a graph 900 of anti-resonant
frequency and resonant frequency as a function of IDT finger pitch
p for XBAR resonators with z-cut lithium niobate piezoelectric
plate thickness ts=400 nm, with front-side dielectric layer
thickness tfd as a parameter. The solid lines 910 and 920 are plots
of the anti-resonance and resonance frequencies, respectively, as
functions of IDT pitch for tfd=0. The dashed lines 912 and 922 are
plots of the anti-resonance and resonance frequencies,
respectively, as functions of IDT pitch for tfd=30 nm. The dash-dot
lines 914 and 924 are plots of the anti-resonance and resonance
frequencies, respectively, as functions of IDT pitch for tfd=60 nm.
The dash-dot-dot lines 916 and 926 are plots of the anti-resonance
and resonance frequencies, respectively, as functions of IDT pitch
for tfd=90 nm. The frequency shifts are approximately linear
functions of tfd.
In FIG. 9, the difference between the resonance and anti-resonance
frequencies is 600 to 650 MHz for any particular values for
front-side dielectric layer thickness and IDT pitch. This
difference is large compared to that of older acoustic filter
technologies, such as surface acoustic wave filters. However, 650
MHz is not sufficient for very wide band filters such as band-pass
filters needed for bands n77 and n79. As described in application
Ser. No. 16/230,443, the front-side dielectric layer over shunt
resonators may be thicker than the front-side dielectric layer over
series resonators to increase the frequency difference between the
resonant frequencies of the shunt resonators and the anti-resonance
frequencies of the series resonators.
Communications devices operating in time-domain duplex (TDD) bands
transmit and receive in the same frequency band. Both the transmit
and receive signal paths pass through a common bandpass filter
connected between an antenna and a transceiver. Communications
devices operating in frequency-domain duplex (FDD) bands transmit
and receive in different frequency bands. The transmit and receive
signal paths pass through separate transmit and receive bandpass
filters connected between an antenna and the transceiver. Filters
for use in TDD bands or filters for use as transmit filters in FDD
bands can be subjected to radio frequency input power levels of 30
dBm or greater and must avoid damage under power.
The insertions loss of acoustic wave bandpass filters is usually
not more than a few dB. Some portion of this lost power is return
loss reflected back to the power source; the rest of the lost power
is dissipated in the filter. Typical band-pass filters for LTE
bands have surface areas of 1.0 to 2.0 square millimeters. Although
the total power dissipation in a filter may be small, the power
density can be high given the small surface area. Further, the
primary loss mechanisms in an acoustic filter are resistive losses
in the conductor patterns and acoustic losses in the IDT fingers
and piezoelectric material. Thus, the power dissipation in an
acoustic filter is concentrated in the acoustic resonators. To
prevent excessive temperature increase in the acoustic resonators,
the heat due to the power dissipation must be conducted away from
the resonators through the filter package to the environment
external to the filter.
In traditional acoustic filters, such as surface acoustic wave
(SAW) filters and bulk acoustic wave (BAW) filters, the heat
generated by power dissipation in the acoustic resonators is
efficiently conducted through the filter substrate and the metal
electrode patterns to the package. In an XBAR device, the
resonators are disposed on thin piezoelectric membranes that are
inefficient heat conductors. The large majority of the heat
generated in an XBAR device must be removed from the resonator via
the IDT fingers and associated conductor patterns.
To minimize power dissipation and maximize heat removal, the IDT
fingers and associated conductors should be formed from a material
that has low electrical resistivity and high thermal conductivity.
Metals having both low resistivity and high thermal conductivity
are listed in the following table:
TABLE-US-00001 Electrical Thermal resistivity conductivity Metal
(10.sup.-6 .OMEGA.-cm) (W/m-K) Silver 1.55 419 Copper 1.70 385 Gold
2.2 301 Aluminum 2.7 210
Silver offers the lowest resistivity and highest thermal
conductivity but is not a viable candidate for IDT conductors due
to the lack of processes for deposition and patterning of silver
thin films. Appropriate processes are available for copper, gold,
and aluminum. Aluminum offers the most mature processes for use in
acoustic resonator devices and potentially the lowest cost, but
with higher resistivity and reduced thermal conductivity compared
to copper and gold. For comparison, the thermal conductivity of
lithium niobate is about 4 W/m-K, or about 2% of the thermal
conductivity of aluminum. Aluminum also has good acoustic
attenuation properties which helps minimize dissipation.
The electric resistance of the IDT fingers can be reduced, and the
thermal conductivity of the IDT fingers can be increased, by
increasing the cross-sectional area of the fingers to the extent
possible. As described in conjunction with FIG. 4, unlike SAW or
AlN BAW, for XBAR there is little coupling of the primary acoustic
mode to the IDT fingers. Changing the width and/or thickness of the
IDT fingers has minimal effect on the primary acoustic mode in an
XBAR device. This is a very uncommon situation for an acoustic wave
resonator. However, the IDT finger geometry does have a substantial
effect on coupling to spurious acoustic modes, such as higher order
shear modes and plate modes that travel laterally in the
piezoelectric diaphragm.
FIG. 10 is a chart illustrating the effect that IDT finger
thickness can have on XBAR performance. The solid curve 1010 is a
plot of the magnitude of the admittance of an XBAR device with the
thickness of the IDT fingers tm=100 nm. The dashed curve 1030 is a
plot of the magnitude of the admittance of an XBAR device with the
thickness of the IDT fingers tm=250 nm. The dot-dash curve 1020 is
a plot of the magnitude of the admittance of an XBAR device with
the thickness of the IDT fingers tm=500 nm. The three curves 1010,
1020, 1030 have been offset vertically by about 1.5 units for
visibility. The three XBAR devices are identical except for the
thickness of the IDT fingers. The piezoelectric plate is lithium
niobate 400 nm thick, the IDT electrodes are aluminum, and the IDT
pitch is 4 microns. The XBAR devices with tm=100 nm and tm=500 nm
have similar resonance frequencies, Q-factors, and
electromechanical coupling. The XBAR device with tm=250 nm exhibits
a spurious mode at a frequency near the resonance frequency, such
that the resonance is effectively split into two low Q-factor, low
admittance peaks separated by several hundred MHz. The XBAR with
tm=250 nm (curve 1030) may not be useable in a filter.
FIG. 11 is a chart illustrating the effect that IDT finger width w
can have on XBAR performance. The solid curve 1110 is a plot of the
magnitude of the admittance of an XBAR device with the width of the
IDT fingers w=0.74 micron. Note the spurious mode resonance at a
frequency about 4.9 GHz, which could lie within the pass-band of a
filter incorporating this resonator. Such effects could cause an
unacceptable perturbation in the transmittance within the filter
passband. The dashed curve 1120 is a plot of the magnitude of the
admittance of an XBAR device with the width of the IDT fingers
w=0.86 micron. The two resonators are identical except for the
dimension w. The piezoelectric plate is lithium niobate 400 nm
thick, the IDT electrodes are aluminum, and the IDT pitch is 3.25
microns. Changing w from 0.74 micron to 0.86 micron suppressed the
spurious mode with little or no effect on resonance frequency and
electromechanical coupling.
Given the complex dependence of spurious mode frequency and
amplitude on diaphragm thickness ts, IDT metal thickness tm, IDT
pitch p and IDT finger width w, the inventors undertook an
empirical evaluation, using two-dimensional finite element
modeling, of a large number of hypothetical XBAR resonators. For
each combination of diaphragm thickness ts, IDT finger thickness
tm, and IDT pitch p, the XBAR resonator was simulated for a
sequence of IDT finger width w values. A figure of merit (FOM) was
calculated for each value of finger width w to estimate the
negative impact of spurious modes. The FOM is calculated by
integrating the negative impact of spurious modes across a defined
frequency range. The FOM and the frequency range depend on the
requirements of a particular filter. The frequency range typically
includes the passband of the filter and may include one or more
stop bands. Spurious modes occurring between the resonance and
anti-resonance frequencies of each hypothetical resonator were
given a heavier weight in the FOM than spurious modes at
frequencies below resonance or above anti-resonance. Hypothetical
resonators having a minimized FOM below a threshold value were
considered potentially "useable", which is to say probably having
sufficiently low spurious modes for use in a filter. Hypothetical
resonators having a minimized cost function above the threshold
value were considered not useable.
FIG. 12 is a chart 1200 showing combinations of IDT pitch and IDT
finger thickness that may provide useable resonators. This chart is
based on two-dimensional simulations of XBARs with lithium niobate
diaphragm thickness ts=400 nm, aluminum conductors, and front-side
dielectric thickness tfd=0. XBARs with IDT pitch and thickness
within shaded regions 1210, 1215, 1220, 1230 are likely to have
sufficiently low spurious effects for use in filters. For each
combination of IDT pitch and IDT finger thickness, the width of the
IDT fingers was selected to minimize the FOM. The black dot 1240
represents an XBAR used in a filter to be discussed subsequently.
Usable resonators exist for IDT finger thickness greater than or
equal to 340 nm and less than or equal to 1000 nm.
As previously discussed, wide bandwidth filters using XBARs may use
a thicker front-side dielectric layer on shunt resonators than on
series resonators to lower the resonance frequencies of the shunt
resonators with respect to the resonance frequencies of the series
resonators. The front-side dielectric layer on shunt resonators may
be as much as 150 nm thicker than the front side dielectric on
series resonators. For ease of manufacturing, it may be preferable
that the same IDT finger thickness be used on both shunt and series
resonators.
FIG. 13 is another chart 1300 showing combinations of IDT pitch and
IDT finger thickness that may provide useable resonators. This
chart is based on simulations of XBARs with lithium niobate
diaphragm thickness=400 nm, aluminum conductors, and tfd=100 nm.
XBARs having IDT pitch and thickness within shaded regions 1310,
1320, 1330 are likely to have sufficiently low spurious effects for
use in filters. For each combination of IDT pitch and IDT finger
thickness, the width of the IDT fingers was selected to minimize
the FOM. The black dot 1340 represents an XBAR used in a filter to
be discussed subsequently. Usable resonators exist for IDT finger
thickness greater than or equal to 350 nm and less than or equal to
900 nm.
Assuming that a filter is designed with no front-side dielectric
layer on series resonators and 100 nm of front-side dielectric on
shunt resonators, FIG. 12 and FIG. 13 jointly define the
combinations of metal thickness and IDT pitch that result in
useable resonators. Specifically, FIG. 12 defines useable
combinations of metal thickness and IDT pitch for series resonators
and FIG. 13 defines useable combinations of metal thickness and IDT
for shunt resonators. Since only a single metal thickness is
desirable for ease of manufacturing, the overlap between the ranges
defined in FIG. 12 and FIG. 13 defines the range of metal
thicknesses for filter using a front-side dielectric to shift the
resonance frequency of shunt resonator. Comparing FIG. 12 and FIG.
13, IDT aluminum thickness between 350 nm and 900 nm (350
nm.ltoreq.tm.ltoreq.900 nm) provides at least one useable value of
pitch for both series and shunt resonators.
FIG. 14 is another chart 1400 showing combinations of IDT pitch and
IDT finger thickness that may provide useable resonators. The chart
is comparable to FIG. 12 with copper, rather than aluminum,
conductors. FIG. 14 is based on simulations of XBARs with lithium
niobate diaphragm thickness=400 nm, copper conductors, and tfd=0.
XBARs having IDT pitch and finger width within shaded regions 1410,
1420, 1430, 1440 are likely to have sufficiently low spurious
effects for use in filters. For each combination of IDT pitch and
IDT finger thickness, the width of the IDT fingers is selected to
minimize the FOM. Usable resonators exist for IDT finger thickness
greater than or equal to 340 nm and less than or equal to 570 nm,
and for IDT finger thickness greater than or equal to 780 nm and
less than or equal to 930 nm.
FIG. 15 is another chart 1500 showing combinations of IDT pitch and
IDT finger thickness that may provide usable resonators. This chart
is based on simulations of XBARs with lithium niobate diaphragm
thickness=400 nm, copper conductors, and tfd=100 nm. XBARs having
IDT pitch and finger thickness within shaded regions 1510, 1520 are
likely to have sufficiently low spurious effects for use in
filters. For each combination of IDT pitch and IDT finger
thickness, the width of the IDT fingers is selected to minimize the
cost function. IDT finger thickness greater than or equal to 340 nm
and less than or equal to 770 nm.
Assuming that a filter is designed with no front-side dielectric
layer on series resonators and 100 nm of front-side dielectric on
shunt resonators, FIG. 14 and FIG. 15 jointly define the
combinations of metal thickness and IDT pitch that result in
useable resonators. Specifically, FIG. 14 defines useful
combinations of metal thickness and IDT pitch for series resonators
and FIG. 15 defines useful combinations of metal thickness and IDT
pitch for shunt resonators. Since only a single metal thickness is
desirable for ease of manufacturing, the overlap between the ranges
defined in FIG. 14 and FIG. 15 defines the range of metal
thicknesses for filter using a front-side dielectric to shift the
resonance frequency of shunt resonator. Comparing FIG. 14 and FIG.
15, IDT copper thickness between 340 nm and 570 nm provides at
least one useable value of pitch for series and shunt
resonators.
Charts similar to FIG. 12, FIG. 13, FIG. 14, and FIG. 15, can be
prepared for other values of front-side dielectric thickness, and
other conductor materials such as Gold.
FIG. 16 is a chart 1600 showing combinations of IDT pitch and IDT
finger thickness that may provide useable resonators on different
thickness diaphragms. The shaded regions 1610, 1615, 1620 define
useable combinations of IDT pitch and aluminum IDT thickness for a
diaphragm thickness of 500 nm. The areas enclosed by solid lines,
such as line 1630, define useable combinations of IDT pitch and
aluminum IDT thickness for a diaphragm thickness of 400 nm. The
solid lines are the boundaries of the shaded areas 1210, 1215, and
1220 of FIG. 12. The areas enclosed by dashed lines, such as line
1640, define useable combinations of IDT pitch and aluminum IDT
thickness for a diaphragm thickness of 300 nm.
Although the combinations of IDT thickness and pitch that result in
useable resonators on 500 nm diaphragms (shaded regions 1610, 1615,
1620), 400 nm diaphragms (regions enclosed by solid lines), and 300
nm diaphragms (regions enclosed by dashed lines) are not identical,
the same general trends are evident. For diaphragm thicknesses of
300, 400, and 500 nm, useable resonators may be made with IDT metal
thickness less than about 0.375 times the diaphragm thickness.
Further, for diaphragm thicknesses of 300, 400, and 500 nm, useable
resonators may be made with IDT aluminum thickness greater than
about 0.85 times the diaphragm thickness and up to at least 1.5
times the diaphragm thickness. Although not shown in FIG. 16, it is
believed that the conclusions drawn from FIG. 12 to FIG. 15 can be
scaled with diaphragm thickness. For aluminum IDT conductors, the
range of IDT thickness that will provide useful resonators is given
by the formula 0.85.ltoreq.tm/ts.ltoreq.2.5. For filters using a
front-side dielectric to shift the resonance frequency of shunt
resonators, the range of aluminum IDT thickness that will provide
useful resonators is given by the formula
0.875.ltoreq.tm/ts.ltoreq.2.25. For copper IDT conductors, the
range of IDT thickness that will provide useful resonators is given
by the formula 0.85.ltoreq.tm/ts.ltoreq.1.42 or the formula
1.95.ltoreq.tm/ts.ltoreq.2.325. For filters using a front-side
dielectric to shift the resonance frequency of shunt resonators,
the range of aluminum IDT thickness that will provide useful
resonators is given by the formula
0.85.ltoreq.tm/ts.ltoreq.1.42.
Experimental results indicate that thin IDT fingers (i.e.
tm/ts.ltoreq.0.375) cannot adequately transport heat out of the
resonator area and IDTs with such thin IDT fingers are unsuitable
for high power applications. Thick IDT conductors (i.e.
tm/ts.gtoreq.0.85) provide greatly improved heat transport.
Experimental results indicate that filters using XBAR resonators
with 500 nm aluminum IDT fingers and 400 nm diaphragm thickness
(tm/ts=1.25) can tolerate 31 dBm CW (continuous wave) RF power
input at the upper edge of the filter passband (commonly the
frequency with the highest power dissipation within a filter
passband).
In addition to having high thermal conductivity, large
cross-section, IDT fingers and a reasonably small aperture, the
various elements of an XBAR filter may be configured to maximize
heat flow between the diaphragms and the environment external to
the filter package. FIG. 17 is a cross-sectional view of a portion
of an XBAR (detail D as defined in FIG. 1). The piezoelectric plate
110 is a single-crystal layer of piezoelectric material. A back
side of the piezoelectric plate 110 is bonded to a substrate 120. A
dielectric bonding layer 1730 may be present between the
piezoelectric plate 110 and the substrate 120 to facilitate bonding
the piezoelectric plate and substrate using a wafer bonding
process. The bonding layer may typically be SiO2. A portion of the
piezoelectric plate 110 forms a diaphragm spanning a cavity 140 in
the substrate 120.
An IDT (130 in FIG. 1) is formed on the front side of the
piezoelectric plate 110.
The IDT includes two bus bars, of which only bus bar 134 is shown
in FIG. 17, and a plurality of interleaved parallel fingers, such
as finger 136, that extend from the bus bars onto a portion of the
piezoelectric plate 110 forming the diaphragm spanning the cavity
140. A conductor 1720 extends from the bus bar 134 to connect the
XBAR to other elements of a filter circuit. The conductor 1720 may
be overlaid with a second conductor layer 1725. The second
conductor layer may provide increased electrical and thermal
conductivity. The second conductor layer 1725 may serve to reduce
the electrical resistance of the connection between the XBAR 100
and other elements of the filter circuit. The second conductor
layer may be the same or different material than the IDT 130. For
example, the second conductor layer 1725 may also be used to form
pads for making electrical connections between the XBAR chip to
circuitry external to the XBAR. The second conductor layer 1725 may
have a portion 1710 extending onto the bus bar 134.
As previously discussed, the metal conductors of the IDT (and the
second conductor layer where present) provide a primary mechanism
for removing heat from an XBAR device as indicated by the bold
dashed arrows 1750, 1760, 1770. Heat generated in the XBAR device
is conducted along the IDT fingers (arrow 1750) to the bus bars. A
portion of the heat is conducted away from the bus bars via the
conductor layers 1720, 1725 (arrows 1760). Another portion of the
heat may pass from the bus bars through the piezoelectric plate 110
and the dielectric layer 1730 to be conducted away through the
substrate 120 (arrow 1770).
To facilitate heat transfer from the conductors to the substrate,
at least portions of the bus bars extend off of the diaphragm onto
the part of the piezoelectric plate 110 that is bonded to the
substrate 120. This allows heat generated by acoustic and resistive
losses in the XBAR device to flow through the parallel fingers of
the IDT to the bus bars and then through the piezoelectric plate to
the substrate 120. For example, in FIG. 3, the dimension wbb is the
width of the bus bar 134 and the dimension wol is the width of the
portion of the bus bar 134 that overlaps the substrate 120. wol may
be at least 50% of wbb. The bus bars may extend off of the
diaphragm and overlap the substrate 120 along the entire length
(i.e. the direction normal to the plane of FIG. 3) of the IDT.
To further facilitate heat transfer from the conductors to the
substrate, a thickness of the bonding layer 1730 may be minimized.
Presently, commercially available bonded wafer (i.e. wafers with a
lithium niobate or lithium tantalate film bonded to a silicon
wafer) have an intermediate SiO2 bonding layer with a thickness of
2 microns. Given the poor thermal conductivity of SiO2, it is
preferred that the thickness of the bonding layer be reduced to 100
nm or less.
The primary path for heat flow from a filter device to the outside
world is through the conductive bumps that provide electrical
connection to the filter. Heat flows from the conductors and
substrate of the filter through the conductive bumps to a circuit
board or other structure that acts as a heat sink for the filter.
The location and number of conductive bumps will have a significant
effect on the temperature rise within a filter. For example,
resonators having the highest power dissipation may be located in
close proximity to conductive bumps. Resonators having high power
dissipation may be separated from each other to the extent
possible. Additional conductive bumps, not required for electrical
connections to the filter, may be provided to improve heat flow
from the filter to the heat sink.
FIG. 18 is a schematic diagram of an exemplary high-power XBAR
band-pass filter for band n79. The circuit of the band-pass filter
1800 is a five-resonator ladder filter, similar to that of FIG. 5.
Series resonators X1 and X5 are each partitioned into two portions
(X1A/B and X5A/B, respectively) connected in parallel. Shunt
resonators X2 and X4 are each divided into four portions (X2A/B/C/D
and X4A/B/C/D, respectively) that are connected in parallel.
Dividing the resonators into two or four portions has the benefit
of reducing the length of each diaphragm. Reducing the diaphragm
length is effective to increase the mechanical strength of the
diaphragm.
FIG. 19 shows an exemplary layout 1900 for the band-pass filter
1800. In this example, the resonators are arranged symmetrically
about a central axis 1910. The signal path flows generally along
the central axis 1910. The symmetrical arrangement of the
resonators reduces undesired coupling between elements of the
filter and improves stop-band rejection. The length of each of the
resonators is arranged in the direction normal to the central axis.
The two portions of series resonators X1A-B and X5A-B are arranged
in-line along the direction normal to the central axis. These
resonators would be divided into more than two portions arranged in
the same manner. The series resonator X3 could be divided not two
or more portions. The shunt resonators are divided into four
portion X2A-D and X4A-D, with the portions disposed in pairs on
either side of the central axis 1910. Positioning the shunt
resonator segments in this manner minimizes the distance between
the center of each resonator portion and the wide ground conductors
at the top and bottom (as seen in FIG. 19) of the device.
Shortening this distance facilitates removing heat from the shunt
resonator segments. Shunt resonators can be divided into an even
number of portions, which may be two, four (as shown), or more than
four. In any case, the half of the portions are positioned on
either side of the central axis 1910. In other filters, the input
port IN and the output port OUT may also be disposed along the
central axis 1910.
FIG. 20 is a chart 2000 showing measured performance of the
band-pass filter 1800. The XBARs are formed on a Z-cut lithium
niobate plate. The thickness is of the lithium niobate plate is 400
nm. The substrate is silicon, the IDT conductors are aluminum, the
front-side dielectric, where present, is SiO2. The thickness tm of
the IDT fingers is 500 nm, such that tm/ts=1.25. The other physical
parameters of the resonators are provided in the following table
(all dimensions are in microns; p=IDT pitch, w=IDT finger width,
AP=aperture, L=length, and tfd=front-side dielectric
thickness):
TABLE-US-00002 Series Resonators Shunt Resonators Parameter X1* X3
X5* X2** X4** P 3.75 3.75 3.75 4.12 4.12 w 1.01 0.86 1.10 0.84 0.93
AP 44 37 41 58 57 L 350 420 350 350 350 tfd 0 0 0 0.100 0.100 *Each
of 2 sections **Each of 4 sections
The series resonators correspond to the filled circle 1240 in FIG.
12, and the shunt resonators correspond to the filled circle 1340
in FIG. 13.
In FIG. 20, the solid line 2010 is a plot of S(1,2), which is the
input-output transfer function of the filter, as a function of
frequency. The dashed line 2020 is a plot of S(1,1), which is the
reflection at the input port, as a function of frequency. The
dash-dot vertical lines delimit band N79 from 4.4 to 5.0 GHz and
the 5 GHz Wi-Fi band from 5.17 GHz to 5.835 GHz. Both plots 2010,
2020 are based on wafer probe measurements having 50-ohm
impedance.
FIG. 21 is a chart 2100 showing measured performance of the band
N79 band-pass filter 1800 over a wider frequency range. In FIG. 21,
the solid line 2110 is a plot of S(1,2), which is the input-output
transfer function of the filter, as a function of frequency. The
dashed line 2120 is a plot of S(1,1), which is the reflection at
the input port, as a function for frequency. Both plots 2110, 2120
are based on wafer probe measurements corrected for 50-ohm
impedance.
Closing Comments
Throughout this description, the embodiments and examples shown
should be considered as exemplars, rather than limitations on the
apparatus and procedures disclosed or claimed. Although many of the
examples presented herein involve specific combinations of method
acts or system elements, it should be understood that those acts
and those elements may be combined in other ways to accomplish the
same objectives. With regard to flowcharts, additional and fewer
steps may be taken, and the steps as shown may be combined or
further refined to achieve the methods described herein. Acts,
elements and features discussed only in connection with one
embodiment are not intended to be excluded from a similar role in
other embodiments.
As used herein, "plurality" means two or more. As used herein, a
"set" of items may include one or more of such items. As used
herein, whether in the written description or the claims, the terms
"comprising", "including", "carrying", "having", "containing",
"involving", and the like are to be understood to be open-ended,
i.e., to mean including but not limited to. Only the transitional
phrases "consisting of" and "consisting essentially of",
respectively, are closed or semi-closed transitional phrases with
respect to claims. Use of ordinal terms such as "first", "second",
"third", etc., in the claims to modify a claim element does not by
itself connote any priority, precedence, or order of one claim
element over another or the temporal order in which acts of a
method are performed, but are used merely as labels to distinguish
one claim element having a certain name from another element having
a same name (but for use of the ordinal term) to distinguish the
claim elements. As used herein, "and/or" means that the listed
items are alternatives, but the alternatives also include any
combination of the listed items.
* * * * *